gotta admit the context here (Linux kernel) makes one wonder if they should be doing this
http://www.openwall.com/lists/kernel-hardening/2018/04/15/1 …
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TL;DR: they relied on first 2 slots of pt_regs overlapping with callee argument area to pass syscall args, and syscalls that made tail-calls happily clobbered the saved pt_regs.
yikes!
Nobody noticed until we ported musl to sh because all the existing users were stuck on CodeSourcery forks of gcc 4.5 or whatever that didn't do tail calls. :-P
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